Making products of iron or iron alloys



Patented Nov. 12, 1946 MAKING PRODUCTS OF IRON OR IRON ALLOYS Lyman Fiske Whitney, Cambridge, Mass., assignor to Isthmian Metals, vInc., Boston, Mass, a corporation of Massachusetts No Drawing. Application August 16, 1944, Serial No. 549,807

9 Claims. ('01. 75-22) This invention relates to the fabrication of articles made of iron or iron alloys, by pressing and sintering starting material which optionally has the final composition of the fabricated article. This starting material is in loose powder form. r

According to this invention, it is possible to make an iron body which is so soft and ductile, that it can be substituted for copper, brass, Babs bitt metal, and other alloys which are used to make bearings for shafts and for other purposes.

Another important use is to make a bandfor shells and other projectiles, to replace the band not exceed 1% of the volume of the iron powder. I

I have found that carbon can be introduced into the above-mentioned starting iron powder, up to a maximum of substantially 0.15% by weight of the iron powder. Hence, I can make the rotation band of a steel in-which the total weight of the non-iron and non-carbon ingredients and of. the carbon, excluding oxide inclusions, water and said gases, is from 0.45% to 0.65% of the weight of the iron. This carbon can be inwhich is now made of copper alloy. Such band high tensile strength combined with good du'ctility, and with final linear dimensions which can be held to close tolerances, thus eliminating or minimizing subsequent machining to final dimensions.

Other objects will be stated in the annexed description.

It is important in many aspects of my invention, to use a starting iron powder of special characteristics. This starting iron powder is preferably made by electro-deposition, using an iron anode and an electrolytic bath according to usual practice, or it may be made by the reduction of iron oxide which has been made by oxidizing relatively pure iron or relatively pure iron salts. iron powder contains minute percentages of various impurities. Some of these impurities may be dissolved in the iron. In addition, such iron has impurities in the form of inclusions which are not dissolved in the iron. Inclusions are generally in the form of oxides or oxysulphides of the respective elements, generally iron oxide. Irrespective of the method of making the iron powder, it is preferred to use an iron powder in which the sum total of all the non-iron ingredients other than carbon, including dissolved impurities but excluding inclusions, and also excluding water and adsorbed gases and absorbed gases, does not exceed 0.3% to 0.5% of the weight of the iron powder, the lower figure of 0.3% being preferred.

It IS preferred that the total volume of the impurities which are present as inclusions, should Investigation has shown that such troducedinto the starting iron powder by heating said iron powder in a suitable earburizing atmosphere, such as methane, propane and the like, at the usual carburizing temperature of about l200 F.-1700 F., using the special precautions which are later stated herein. The carbon can also be introduced into the starting iron powder by intermixing said iron powder with graphite. In either manner, I can introduce a larger percentage of carbon than 0.15% into the iron powder, thus making a steel which may have as much as 1.5% of carbon.

This starting iron powder is thoroughly annealed in order to relieve strains which have been caused by work-hardening such powder and to eliminate embrittlement. Work-hardening can be caused by cold-working, as in a ball-mill. Embrittlement can also be caused by the absorption or adsorption of hydrogen, during electrodepo sition of the iron powder. The special annealing test is later described.

In order to make an iron shell-band, the starting material may be a loose and freely-flowing iron powder which p eferably has the characteristics above-mentioned.

This charge of loose iron powder or loose steel powder is subjected to cold-pressing at a relatively light pressure of 15-40 tons per square inch. It is preferable, that the initial density which is secured by this first cold-pressing step should be relatively low and should not exceed 6-6.8, for example. It should not exceed 7.

If the density of the first cold-pressed body is made too high, cracks will be formed in the cold-pressed body, and these cracks will not be completely closed or sealed in the subsequent successive operations of sintering, pressing and sintering. The result will be the presence of cracks in the final body or product, and such cracks are objectionable. These cracks may appear in the surface and/or in the interior of the body. If the density of the first cold-pressed body is too low, as for example below 4.6, the cold-pressed body cannot be handled without 3 breakage or chipping. If the density of the first cold-pressed body is between 4.6 and 5.5, for example, such body will shrink too much during the subsequent first sintering operation. In addition, the variation in such shrinkage will be too great in respective cold-pressed bodies,

- and the shrinkage will not be uniform in the respective cold-pressed body. Hence, it is preferred that the density of the first cold-pressed body should be between 5.5 and 6.8, in order to avoid cracking and chipping, and to produce a definite and uniform shrinkage during the first sintering operation, so that all the dimensions of the cold-pressed body will be diminished by approximately the same percentage. ably, the density of the first cold-pressed body is 6.3. This first cold-pressed body is sintered, without using pressure during the sintering, at 1600 F.- 2000 F., during a sintering period of twenty minutes to one hour, and even up to three hours. As a specific example, the sintering temperature may be 1660 F., and the sintering period may be thirty minutes. As another specific example, the sintering temperature may be 1800 F. and the. sintering period may be two hours.

This sintering is done in an inert or non-oxidizing atmosphere. For example, dry oxygen-free hydrogen or dry cracked oxygen-free ammonia gas provide a satisfactory sintering atmosphere for sintering an iron compact. When ammonia gas is cracked, nitrogen and hydrogen are produced. Dry and oxygen-free nitrogen and/or argon can also be used. If a cold-pressed steel briquette or a mixture of iron and carbon is thus sintered, the atmosphere may be a dry and oxygen-free mixture of hydrogen and methane or other stabilizing gas, intermixed in suitable proportion to prevent the sintering atmosphere from carburizing or decarburizing during the sintering, as elsewhere more fully stated herein.

The sintered body should be cooled in the sintering atmosphere, until the sintered body will not oxidize when it is located in the air or in other oxidizing atmosphere. Hence, the sintered band is kept free from oxidation.

After the first sintering the compact is again cold-pressed at a higher pressure to obtain the desired density.

Normally, a second sintering operation is useful after the second cold-pressing, in order to elimihate or reduce the work-hardening which results from the second cold-pressing and in order to improve the physical properties of the body. The second sintering is performed under the same general conditions as the first sintering.- The temperature of the second sintering can be 1500". F.2000 F., the sintering period can be twenty minutes to one hour or more, and the sintered band is preferably allowed to cool slowly in the sintering atmosphere to 400 F.-500 F., as previously stated. 1

The upper limit of the sintering temperature may -be as high as 2400 F., in sintering either iron or steel, except that in sintering steel the sintering temperature should preferably be kept at least 100 F. below the melting point of the steel, which is being sintered. The melting point of the steel depends upon its carbon content.

Shell-bands made from said iron powder, according to the multiple-step method previously described, can be cold-swaged into the groove of the shell or other projectile, using the same method and apparatus which are now used for cold-swaging copper-alloy hands into said groove.-

Prefer- The iron band can generally be cut and bent back on itself through an angle of without breaking, even after the band has been coldswaged into the groove in a projectile.

In making a shell-band, the iron powder may be used with or without admixture with other ingredients such as binders, lubricants, etc.

When iron powderof the aforesaid required properties is used as the starting material, it is annealed in hydrogen at 1300 F.-l500 R, if annealing is required to secure the proper softness, and the resultant cake of annealed material is then gently disintegrated. Said cake may be disintegrated in a hammer mill, using suitable precautions to prevent excessive hardening. The hydrogen is preferably carefully dried so that it is free from water vapor.

Commercial so-called pure hydrogencontains substantial traces of oxygen and water vapor. In order preferably to remove the oxygen, the commercial hydrogen is led over heated copper catalyst at about 1000 F.-1200 F., in order to cause the oxygen impurity to combine with some of the hydrogen, to produce water vapor. The hydrogen is then passed through a series of driers or desiccating chambers, in order to remove substantially all of the water vapor.

One test that can be made to determine whether the powder has been thoroughly annealed, is to view a properly prepared section of the powder under a microscope to ascertain that the minimum mean grain size per ferrite grain is at least .0002 square millimeter and that the volume of oxide inclusion does not exceed 1%, as elsewhere stated herein.

A further test is to determine whether the powder is sufficiently free from water and adsorbed and absorbed gases. For this purpose the powder or low density compact which has been made by cold-pressing said powder, may be tested by heating in said substantially dry oxygenfree hydrogen, at 1800" F. during a period of two hours. If a compact is used, its density may be just sufiicient to make it coherent. The powder should show a loss of weight during the two hours of not more than 0.7%. Preferably the loss of weightshould be less than 0.4%. If this powder has less than 0.3 to 0.5% of impurities by weight, exclusive of carbon and inclusions and water and adsorbed and adbsorbed gases, and has not been cold worked or otherwise strain hardened, it is a soft powder suitable for use as one of my preferred starting materials.

According to one method which is within the scope of my invention, the iron powder is mixed with the proper proportion of graphite, a briquette is made of this mixture by cold-pressing at 15-40 tons per square inch, and this briquette is then heated and thus sintered in an atmosphere which prevents any injurious loss of the graphite during the sintering operation. Preferably this sintering operation is carried out at a temperature of 2000 F. and for a period of from one to three hours, in an atmosphere of methane and hydroen.

The methane can be replaced by propane or other stabilizing gas.

It is desired to combine all the graphite with the iron powder. Hence the proportion of graphite is the desired final proportion of combined carbon in the final steel body. The proportion of methane in the sintering atmosphere depends upon the sintering temperature and the final proportion of combined carbon which it is desired tom" have in the final steel product. The methane" the desired limit. Hence the proportion of methane is selected so that the briquette neither loses nor gains carbon during the sintering, and the proportion of combined carbon in the final steel material'is determined by the proportion of graphite in the briquette.

As elsewhere explained herein, it is very important to provide a sintering atmosphere during carburization, which is free from water vapor and oxygen. Hence the commercial hydrogen and stabilizing gas mixture is purified by removing the oxygen, and water vapor is also removed, as further explained herein.

After the first sintering, when the iron has combined with the graphite, the briquette is again cold-pressed at a higher pressure to obtain the desired density, and then resintered. If the sintering temperature is 1800 F. in the second sintering operation, it will be necessary to use an atmosphere which has a larger percentage of methane than during the first sintering operation at 2000 F. If it is desired to produce a hardened steel body, the sintered steel body is promptly quenched in water, oil or brine from the sintering temperature immediately at the completion of the last sintering operation. The quenched body can then be drawn to the desired softness.

In practicingthe improved method for making a steel body describedabove, I prefer to use the pure iron powder having the softness and freedom from water and absorbed and adsorbed gases and the minimum mean area per ferrite grain as previously described.

Such powder, mixed with graphite, makes an excellent starting material for making steel bodies. In processing such material by the improved method just described, small cracks and voids which remain after the first cold-pressing operation and after the first sintering operation, are sealed either during the second cold-pressing operation or by the second cold-pressing combined with the second sintering operation.

Furthermore the freedom of such pure iron powder from adsorbed and absorbed gases and water is an important factor in preventing decarburization of the body during the sintering process.

Thus, this aforesaid pure iron powder is a very important feature of this invention.

After sintering iron which is combined with any substantial proportion of carbon, either prior to or during the sintering operation, according to any method disclosed herein, it is important that the sintered body, at the completion of the sintering period, be protected against decarburization and oxidation during the cooling period. If the sintering atmosphere comprises hydrogen and a stabilizing gas, it is preferable quickly to cool the sintered compact in the sintering atmosphere. Hence, it is preferable at the end of each sintering operation, to move the sintered compact to the cool zone of the sintering furnace, while still in contact in said cool zone with the hydrogenmethane sintering atmosphere. The compact should be cooled in the sintering atmosphere from the sintering temperature, down to about 1100 F. in less than 10 minutes. It said time of cooling from the sintering temperature to 1100 F. is relatively slow (or the order of 20 minutes or more) the compact will lose a very substantial amount of carbon and the carbon thatis left in the compact will be unevenly distributed throughout the compact. When the aforesaid cooling rate is of the order of one half minute to two minutes, either no carbon is lost or the loss is small. The reason for the loss of carbon when the aforesaid cooling period is relatively long is as follows: As the temperature drops, the percentage of methane must be increased to prevent decarburization until the temperature becomesso low that the rate of piece has been cooled to 1100 F., the further time required to cool said piece to room temperature is unimportant.

On the other hand if the briquette is to be repressed after being sintered, the cooling rate after said sinter should not be too rapid because if the cooling period is too short (for example if the briquette is quenched) the structure of the sintered compact will be injuriously hardened, thus making it impossible to shape the sintered compact by means of the second cold-pressing operation to the desired high final density. The cooling time from the sintering temperature to 1100 F. must be slow enough so that the resulting structure of the sintered material is pearlitic.

Another method which I use in sintering a pressed compact comprising a mixture of iron, carbon and lubricant, is to heat the compact in a zone of the furnace through which a non-oxidizing atmosphere is passing and where the temperature does not exceed 1100 F., in order to drive oil the lubricant, holding the compact in this zone for a time suflicient to drive off substantially all the lubricant. In the case of a combustible atmosphere which is burned at the outlet of the furnace, this time can be determined by the change in color of the flame of the atmosphere which is burning at the outlet of the furnace, and it can be of the order of 20 minutes.

If the atmosphere is hydrogen and a stabilizing gas of the type described herein, the compact may be transferred directly from the zone of the furnace described above to a second zone of the furnace which is at the sintering temperature, as soon as the lubricant has been driven off. In this second zone, the compact may be sintered as previously described.

The compact can also be heated at a temperature not exceeding 1100 F. in an atmosphere of hydrogen or inert gas to drive off the lubricant and the compact can be cooled in that atmosphere after the lubricant has been eliminated. The compact can then be placed directly in the zone of a furnace which is maintained at sintering temperature and through which is passing an atmosphere of hydrogen and a stabilizing gas of the type described, and the compact is sintered as previously stated. After the lubricant has been driven oil, instead of cooling the compact, as aforesaid, the compact can be placed directly in a stabilized hydrogen atmosphere and thereafter sintered as previously described.

As stated, I can use inert gases such as nitrogen or argon as thesintering atmosphere for sinteroxygen and water vapor in sintering such coldpressed bodies. Preferably, the iron powder used as one of the starting materials should be very free from water and adsorbed and absorbed gases. The loss of weight of such powder when heated for two hours in dry and oxygen-free hydrogen at 1800 F. should preferably not exceed 0.4% of the weight of said powder. When such dry and oxygen-free inert gases are used as the sintering atmosphere a rapid cooling rate is not of 0.3% to 0.5% by weight, its gamma range begins at about 1660 F. This material is therefore sintered close toand usually above the lower limit of its gamma range. If the percentage of carbon is increased, the lower limit of the gamma range is decreased.

The maximum tensile strength of iron specimens made according to this invention varied from about 39,000 lbs. per square inch to about 53,000 lbs. per square inch. .The percentage of elongation at breaking point under tensile stress varied between about 33% to 60%. This shows high ductility and bendability, which are necessary for the purposes of my invention. The density varied from 7.40 to 7.69. The Rockwell F hardness varied from about 55-75. These figures refer to the specimens, after the second sintering.

The use of a very dry and oxygen-free atmosphere is particularly important in the first sintering operation which follows the first pressing operation, where the pressure used in such first pressing operation is relatively low, so that the resulting body has a low density and is therefore porous. The sintering atmosphere penetrates such porous mass, so that water vapor and oxygen will penetrate the interior of the body to oxidize some carbon or iron or other oxidizable material that may be present. One test for determining the substantial absence of oxygen and, water vapor is to heat stainless steel; preferably a steel which has 18% of chromium and 8% of nickel, in said sintering atmosphere, up to the sintering temperature. If said steel does not discolor, this evidences the substantial absence of oxygen and Water vapor, because chromium oxidizes very readily.

In the second pressing operation the pressure should be at least 60 tons per square inch and preferably not more than 90 tons per \square inch.

I have described various embodiments of my invention for illustrative purposes, but numerous changes and omissions and additions can be made without departing from its scope.

I claim:

1. In the art of making a metal article by processing a powdered starting material comprising electrolytically deposited iron the method which comprises cold-pressing said loose powdered starting material to provide a coherent cold-pressed compact, the pressure of the first cold-pressing ing cold-pressed bodies containing iron and car- 'bon but such gases 'must be extremely free of 8 being high enough to produce a unitary compact but not greater than about 40 tons per square inch, sintering said cold-pressed compact in a non-oxidizing atmosphere at a temperature of at least approximately 1600 F. to remove the work-hardening which results from said coldpressing, again cold-pressing the sintered compact at a higher pressure than during the first cold-pressing, the pressure of the second coldpressing being at least about 60 tons per square inch, and again sintering after the second coldpressing at a temperature of at least 1500 F. to substantially remove the work-hardening which results from the second cold-pressing.

2. In the art of making a metal article by processing a powdered starting material comprising iron which has a maximum of substantially 0.5% by weight of non-carbon and non-iron dissolved ingredients other than inclusion material, water and absorbed gases, and containing a maximum of substantially one per cent by volume of inclusion material, and containing a maximum of 0.7% by weight of water and adsorbed and absorbed gases, said iron having a minimum mean -ferrite grain area of substantially 0.0002 square millimeter, the method which comprises coldpressing said loose powdered starting material to provide a coherent cold-pressed compact, the pressure of the first cold-pressing being high enough to produce a unitary compact but not greater than about tons per square inch, sintering said cold-pressed compact in a, non-oxidizing atmosphere at a temperature of at least aping which results from said col -pressing, again proximately 1600 F. to IEIIIOVZFBF cold-pressing the sintered co pact at a higher pressure than during th first cold-pressing, the

pressure of the second cold-pressing being at least about 60 tons per square inch and again sintering after the second cold-pressing at a temperature of at least 1500 F. to substantially remove the work-hardening which results from the second cold-pressing.

s. The method of making a body with iron powder which comprises first shaping the body by compressing the powder, sintering the body,

, again compressing the body at higher pressure and then sintering the body again, characterized in that the pressure of the first compression does not substantially exceed 40 tons per square inch and the pressure of the second compression is at least about 60 tons per square inch and high enough to compress the body to a density of at least 7.4.

4. The method of making a body with iron powder which comprises first shaping the body by compressing the powder, sintering the body,

again compressin the body at higher pressure and then sintering the body again, characterized in that the pressure of the first compression is about 15 tons per square inch to about 40 tons per square inch and the pressure of the second compression is about 60 tons per square inch to about 90 tons per square inch.

5. In the art of making a metal article by processing material comprising powdered iron which comprises first shaping the body by compressing the powder to form a compact, sintering the compact at a temperature of at least about 1600 F. for a period of at least 20 minutes, pressing the compact at higher pressure and then sintering the compact again at a temperature of at least about 1500 F. for a period of at least 20 minutes, characterized in that the first compression is great enough to produce a unitary selfwork -hardensustaining compact and does not substantially exceed 40 tons per square inch and the second compression is at least about 60 tons per square inch.

6. The method of making a body with iron powder which comprises first shaping the body by compressing the powder to form a compact, sinterlng the compact, pressing the compact at higher pressure, and then sintering the compact again, characterized in that the first compression is great enough to produce a unitary self-sustaining compact and does not substantially exceed 40 tons per square inch and the second compression is at least about 60 tons per square inch.

7. The method of making a body with iron powder which comprises first shaping the body by compressing the powder to form a compact, sintering the compact pressing the compact at higher pressure, and then sintering the compact again, characterized in that the first compression is great enough to produce a. unitary self-sustaining compact and does not substantially exceed 40 tons per square inch and the second compression is at least about 60 tons per square inch and the second sintering is effected at a temperature of at least 1500 F.

8. In the art of making a compact by coldpressing and heating the compact within a range 01' slntering temperatures to knit the powder particles together, the method which comprises mixing a lubricant with the powder, cold-pressing the lubricated powder, expelling substantially all the lubricant from the compact with heat at a 10 temperature below said range, sintering the compact, pressing the compact at higher pressure, and then sintering the compact again, the first compression being great enough to produce a unitary self-sustaining compact and not substantially exceeding 40 tons per square inch and the second compression being at least about tonsper square inch and the second sintering being eflected at a temperature of at least 1500" F.

9. In the art 01 making a steel body from powdered material which includes iron and carbon by two cold-pressing steps, each cold-pressing step being followed by a sintering step, the meth. od which comprises cold-pressing said material at a pressure not greater than about 40 tons per square inch to form a briquette, sintering said briquette in an atmosphere which is approximately in equilibrium with the carbon content of the briquette at a temperatur oi. at least approximately 1600 F., and at he conclusion 01' said sintering period cooling the brlquette fast enough to prevent substantial decarburization, repressing said briquette at a pressure of at least about-r60 tons per square inch, resintering the briquette in an atmosphere which is approximately in equilibrium with the carbon content of the briquette at a temperature of at least approximately 1500 F., and at the conclusion of said second sintering period cooling the repressed and resintered steel briquette fast enough to prevent substantial decarburization. 

